AO-5: Approved Programmes

Announced on 28 June 2024

The 5th Announcement of Opportunity (AO-5) for the CHEOPS Guest Observers (GO) Programme opened on 12 March 2024 and closed on 25 April 2024. It is the second call of the first extended mission and covers observing time in the period from 1 October 2024 until 30 September 2025. The available GO share of the science observing time was recently increased from 20% (in the Nominal Mission) to 30% (in the first Extended Mission).

A total of 36 proposals were received in reply to AO-5, requesting 2415 orbits (with each orbit circa 99 minutes in duration). The requests represented 160% of the available science observing time foreseen in the AO-5 observing cycle (circa 1505.5 orbits).

The CHEOPS Time Allocation Committee (TAC) met on 5 - 7 June 2024. Based on the TAC's recommendations, the Director of Science has awarded CHEOPS observing time to the proposals listed in the table below. In the end, 20 proposals were awarded observing time totaling 1640 orbits. Of these, 18 proposals were on exoplanet science and 2 on stellar science. The TAC-recommended allocation of observing time represents up to 109% of the available GO science observing time foreseen in the AO-5 observing cycle.

Succesful proposals will be implemented as GO programmes. Targets that are part of these GO programmes can generally not be included in other observing programmes unless in specific cases. All programmes have been assigned a priority from Priority 1, or P1 (high), to Priority 3, or P3, (low). This priority is taken into account by the automated planning tool used in the weekly/biweekly planning, and is a strong indicator of the likelihood that observations will be scheduled. Observers are reminded that the award of observing time provides no guarantee that the observations can be executed, and that generally more observing timeis awarded than can be physically scheduled to minimise idle time.

Principal Investigators (PIs) of proposals that have been awarded time have been contacted by email, and are required to complete and submit observation requests at their earliest convenience. Guidelines on how to prepare observation requests can be found here. The TAC feedback has been provided to all PIs of proposals.

A fraction of up to 15% of the GO Programme time will remain available to the community to apply for time via the Discretionary Programme (DP), which is foreseen to continue running throughout the mission lifetime. This is in line with a possible over-allocation of up to 124% to facilitate the efficient scheduling of time critical observations.

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* Only the four targets in RV follow-up programmes, one of them shared with another programme.

** Only the target not in common with a higher rank proposal.

Abstracts

Seeking Earths in the shadows: visiting the Lagrangian points of two co-orbital candidates (PI: Olga Balsalobre-Ruza)

Detecting the transit of a long-period, temperate, radial velocity identified planetary signal is (at least regarding personal feelings) like winning the lottery. This is because, even if in the future we will be able to characterise the planetary atmospheres of long-period non-transiting planets through space-based interferometers, the only possibility as of today to perform such studies is by detecting the transit signal. If a planet transits, we can have access to a plethora of properties; the planetary radius, the absolute mass, the orbital architecture, or the atmospheric composition. Here, we aim at confirming the transiting nature of a radial velocity identified temperate planet, part of a multi-planet system with at least another inner planet in 7:2 mean motion resonance. A mono-transit compatible with the radial velocity derived properties and ephemeris of this planet was detected in TESS sector 17. Based on this, the planet is a 12.9 M_earth and 1.7 R_earth planet in the super-Earth regime. The planet has an orbital period of 29.6 days and orbits a K7-M0 dwarf star, inducing an equilibrium temperature of 358 K, thus becoming a warm-to-temperate super-Earth candidate. We propose to observe three transits with CHEOPS in AO5, taking profit of its high-quality photometry and space-based flexibility that will allow us to 1) constrain the transit ephemeris, 2) look for possible TTVs, 3) precisely determine the physical properties, and 4) trigger internal structure studies.

Measuring the nanoflaring activity of bright M dwarfs (PI: Julian Poyatos)

The stochastic nature of stellar flares makes them unlikely to be frequent enough to explain the measured coronal heating of their star. Instead, nanoflares, with energies ranging from 10²² to 10²⁵ erg, have been proposed as alternative contributors due to their higher occurrence rates since they are exponentially more frequent than regular flares (> 10²⁸ erg). However, the mechanisms responsible for their generation are still unclear. Through this filler proposal, we aim to use the exquisite photometric precision of CHEOPS to gather nanoflare statistics from bright M dwarfs and provide new insights on some of the current open questions concerning them. Specifically, we will first determine the nanoflare Flare Frequency Distribution (FFD) deviation from the regular flares FFD, which would indicate that a different reconnection process is at play. We will also test if nanoflaring activity could be the cause of the p-mode oscillations detected on some M dwarf stars. Finally, we will observe partially and fully convective M dwarfs to determine if the presence of a tachocline has an impact on the nanoflaring rate.

A second transit for a radial velocity detected temperate sub-Neptune in a multi-planetary system (PI: Jorge Lillo-Box)

Detecting the transit of a long-period, temperate, radial velocity identified planetary signal is (at least regarding personal feelings) like winning the lottery. This is because, even if in the future we will be able to characterise the planetary atmospheres of long-period non-transiting planets through space-based interferometers, the only possibility as of today to perform such studies is by detecting the transit signal. If a planet transits, we can have access to a plethora of properties; the planetary radius, the absolute mass, the orbital architecture, or the atmospheric composition. Here, we aim at confirming the transiting nature of a radial velocity identified temperate planet, part of a multi-planet system with at least another inner planet in 7:2 mean motion resonance. A mono-transit compatible with the radial velocity derived properties and ephemeris of this planet was detected in TESS sector 17. Based on this, the planet is a 12.9 M_earth and 1.7 R_earth planet in the super-Earth regime. The planet has an orbital period of 29.6 days and orbits a K7-M0 dwarf star, inducing an equilibrium temperature of 358 K, thus becoming a warm-to-temperate super-Earth candidate. We propose to observe three transits with CHEOPS in AO5, taking profit of its high-quality photometry and space-based flexibility that will allow us to 1) constrain the transit ephemeris, 2) look for possible TTVs, 3) precisely determine the physical properties, and 4) trigger internal structure studies.

Abnormally Rapid Transit Timing Variations of WASP-161b: Evidence for Tidal Evolution or the Existence of an Earth-Sized Planet? (PI: Fan Yang)

WASP-161b is a hot Jupiter with reported transit timing variations (TTVs), which provides evidence for a decaying orbital period. The physical origin of the TTVs is not clear as we find that the timing evolution can be explained both by period decay through tidal migration and a multi-planet interaction scenario. We propose to conduct three transit observations, each encompassing seven orbits, utilizing the CHEOPS. CHEOPS will provide the most meticulous timing measurements for WASP-161, enabling us to accurately measure its period decay rate. This will, in turn, result in a better understanding of the tidal dissipation process in hot Jupiters and explain their origins. Moreover, CHEOPS observations are crucial in providing stochastic timing sampling when distinguishing the multiplanet interaction, given that the SSO-Europa and TESS observations have a rough uniform interval of two years. The constraint from the CHEOPS timing significantly narrows down the possible interacting planet's mass range. We expect a timing difference of 12.5 minutes between the period decaying and multiplanet interacting prediction if CHEOPS observations are scheduled in 2025. Based on the photometry precision predicted by the CHEOPS Exposure Time Calculator and CHEOPS outputs of WASP-161b in 2022, this difference is at the significant level of 12 $\sigma$ if the timing uncertainty is the same as the 2022 observations.

Validation of candidate TESS Neptune Desert Planets (PI: Matthew Standing)

Over 1400 exoplanetary systems are known to host planets with orbital periods less than 5 days, however, there is a distinct lack of Neptune sized exoplanets (radii from 3-10 R_Earth) at these short periods. Therefore, this region in parameter space is commonly referred to as the “Neptune desert”. It is not clear what mechanisms lead to the dearth of exoplanets in this region, though many theories have been put forward. it is thought to be caused by physical mechanisms at play during planet formation and/or evolution. With this program we propose to validate 9 planetary candidates identified by the Transiting Exoplanet Survey Satellite (TESS) which appear to lie in this Neptune desert. We will validate the planetary nature of all targets while also improving the radii measurements of 5 targets by 54-83%, and by doing so ascertain whether these candidate planets are indeed members of the Neptune desert.

What is the orbital period of the most massive very young transiting exoplanet, HD 114082 b? (PI: Carlos del Burgo)

HD 114082 is a bright, relatively nearby, very young F-type star in the Scorpius-Centaurus (Sco-Cen) OB Association. It bears a nearly edge-on debris disc (Wahhaj et al. 2016) and two planet candidates. HD 114082 b, a transiting planet confirmed by the Doppler method, is so massive that defies current theoretical planet formation models (Zakhozhay et al. 2022, Engler et al. 2023). The planet produced a single-transit event during the TESS observations and its orbital period was at first estimated using radial velocities. A second transit event was detected in March 2023 by the NGTS survey, which allowed us to establish a mid-transit time difference and a set of plausible orbital periods. One of these is similar but still different from that derived from the radial velocity curve. Constraining the orbital period of young planets solely from radial velocity measurements is very challenging and prone to underestimate uncertainties due to limitations in the stellar activity modelling. The most reliable way of confirming the orbital elements is detecting and analysing new transit events, preferably from space. We propose to use 86.1 CHEOPS orbits to observe HD 114082 during three tentative transit events, in accordance with our orbital period candidates, to confirm and refine the planet ephemeris and improve the characterisation of this stunning planet.

Detecting the Second Transit of a Temperate sub-Neptune to Enable Atmospheric Studies (PI: Victoria DiTomasso)

The majority of planets in our galaxy are intermediate in size between the Earth and Neptune, yet their true nature remains unknown: One physical picture posits they are terrestrial worlds enveloped in H/He, while another imagines that they are migrated ice giants. One promising path to learn their true nature employs spectroscopic characterization of their atmospheres; this, in turn, requires bright exemplars with well-determined orbital periods. HD60779b is a sub-Neptune sized planet on a 30d orbit around a bright G1V star. This planet’s Transmission Spectroscopy Metric (TSM=83) distinguishes it as one of the most spectroscopically available sub-Neptunes, especially among its long-period counterparts, or those around stars hotter than the Sun. HD60779b was discovered via a single TESS transit and followed up via extensive radial velocity monitoring. With only one transit observation, the orbital period of HD60779b is not known with sufficient precision to enable atmospheric studies. Our proposed observations will remedy this.

Characterization of the system K2-155 with CHEOPS (PI: Alejandro Suárez Mascareño)

K2-155 is a low-mass star (K6–M0) that hosts 3 transiting planets. K2-155 b, c, and d are super-Earth/mini-Neptunes with orbital periods of 6.34, 13.85 and 40.72 days, respectively. Planets b and d, with a radii of 1.8 R_⊕ and 1.9 R_⊕, respectively, could be super-Earths or water-worlds, depending on their masses, with planet d orbiting at the inner edge of the habitable zone. Planet c, with a radius of 2.6 R⊕ likely falls into the mini-Neptune category of mini-Neptunes. There is an ongoing effort to obtain precise mass measurements of these planets using the MAROON-X spectrograph. However, the current uncertainty in the planetary radii (of more than 10% in the case of the outer planet) will prevent a precise characterisation of their interior compositions and structures. We propose to carry out a high precision photometric campaign using CHEOPS to refine the radius measurements of the planets in the system down to a 5% precision, that will enable the study of their interior structures. We request 108 hours of observing time (64.8 orbits) to observe three transits of planet b, and two transits of planets c and d. We will use this information to perform a full characterization of the system, including the determination of what formation scenario(s) could produce three low-mass planets with potentially different compositions around the same star.

A High-Precision Primary Eclipse of a “Benchmark” Hierarchical Triple Star System (PI: Daniel Stevens)

We request 2 primary eclipse observations, plus out-of-eclipse baseline, of a bright hierarchical triple stellar system composed of a pulsating primary star that forms an eclipsing binary with the lower-mass secondary and an SB2 with the speckle-resolved tertiary. We will determine its physical parameters and orbital architecture to percent-level precision and accuracy, and in multiple orthogonal ways, to test models of stellar structure, asteroseismology, and dynamical interactions. The extant light curves do not capture a single full primary eclipse event, so the CHEOPS observations are an essential ingredient in out simultaneous analysis of existing data: partial primary eclipses, space-based secondary eclipses and pulsations, RV orbits of the inner SB1 and outer SB2, multi-epoch speckle imaging, and multi-wavelength broad- and narrow-band spectral energy distribution (SED) photometry.

A CHEOPS Light Curve of ε Eridani with Contemporaneous Long-Baseline Optical Interferometry and Extreme Precision Radial Velocity Observations (PI: Rachael Roettenbacher)

Understanding the impact of stellar surfaces to the radial velocities detected is essential to on-going studies aiming to find Earth analogs. These studies are inhibited by no clear method of accounting for the stellar contribution. One potential solution is to image the stellar surface in order to model its radial velocity independently of any observed radial velocities. To explore the potential of this method, we propose to obtain a high-precision CHEOPS light curve of the spotted, planet-hosting star ε Eridani. With these observations, we will reconstruct the stellar surface of ε Eri with a light-curve inversion technique. These observations will be contemporaneous with interferometric observations that will be used to image the stellar surface. This surface will be compared to the CHEOPS surface to strengthen the validity of the individual images. From the surface, we will model the radial velocity signal, which will be compared to a radial velocity curve obtained contemporaneously. Using these new and archival observations, we will remove the signature of the reconstructed stellar surfaces to reveal a clear signal of the known planet as a proof-of-concept that imaged stellar surfaces can aid in the detection of Earth analogs. We will compare these results with the same test done with interferometric images to understand if images from photometry are sufficient or if the more detailed interferometric images are necessary.

Transit Timing Variation Monitoring of 2 planets around a very young star (PI: Anne Dattilo)

We request 118 orbits of CHEOPS observations to continue monitoring the transit timing variations of the planetary system orbiting the star TOI-6109, a young, ∼75 Myr-old Sun-like star in the alpha Persei cluster. Previous CHEOPS observations have confirmed two planets in a near 3:2 mean motion resonance with apparent transit timing variations (priv. comm.). Continued monitoring of the system, over several months, should reveal their masses where previous observations have not. The two planets are the progenitors of the ubiquitous sub-Neptunes that have few compositional constraints. The system’s youth and near-resonant configuration present a unique opportunity to test theoretical models of sub-Neptune formation and evolution.

Confirmation of two transiting planets around pre-main sequence stars with CHEOPS (PI: Sydney Vach)

Studying and characterizing young planets is crucial for exploring the universal mechanisms of planet formation and evolution. We propose to use 42 CHEOPS transits to characterize two newborn planets located in the 3 Myr Taurus-Auriga and 16 Myr Upper Centaurus–Lupus associations. IRAS 04125+2902 b (P = 8.83 d, Rp = 10.9 R_⊕) is the first transiting planet around a 3 Myr old protostar, still residing in its natal disk. Initial RV observations suggest IRAS 04125+2902 b is not a newborn Jovian-sized planet, but rather a small planet progenitor. TIC 88785435 b (P = 10.51 d, Rp = 4.88 R_⊕) is a super-Neptune around a 16 Myr old pre-main-sequence star. This program will probe the optical transit depths of two newborn planets before and after the dissipation of the protoplanetary disk, and preserve their transit ephemeris to enable future atmospheric characterization programs.

Characterizing The Brightest-Host Transiting Habitable Zone Terrestrial Exoplanet (PI: Nicholas Scarsdale)

We propose to use CHEOPS photometry to confirm the period of the Transiting Exoplanet Survey Satellite (TESS) planet candidate TOI4353.01 (TIC176797879) and begin to characterize its transit timing variations (TTVs). TOI4353.01 was initially a duo-transit object that was confirmed with a transit detection in previous CHEOPS observations (priv. comm.) and displayed an ingress that appears to identify its period as 40 days (priv. comm.). This object is a terrestrial-size exoplanet in the habitable zone of its bright host star. It has potential for future mass measurement and even transmission spectroscopy thanks to its bright (for an M star) host. In fact, no other OHZ terrestrial exoplanet has a host star as bright as TOI4353.01. Because of the rarity of terrestrial OHZ exoplanets and the difficulty of discovering more with TESS due to its short sectors, followup of candidates like this one is of critical importance. Our proposed observations will provide essential groundwork for future full TTV characterization of the system as well as support future mass measurement.

CHEOPS' scrutiny of long-period planets: characterization of a warm Jupiter orbiting a Sun-like star (PI: Gaia Lacedelli)

The origin of hot and warm Jupiters is still a challenge for formation and evolutionary models, as several observed differences in the orbital parameters of the two populations cannot be explained by a single evolutionary scenario. To better understand their evolutionary path, a solid sample of both populations is needed, but to date we are still missing a robust statistic for warm Jupiters, especially in the long-period regime, with just a few well-characterized giant planets with periods longer than 20 days. Here, we propose to use CHEOPS to solve the period of the TESS duo-transiting candidate TOI-5626.01, a long-period Jupiter-size planet orbiting a Sun-like star. The 2 not-consecutive transits observed by TESS, almost 2 years apart, imply 33 period aliases, spanning from 21 to 716 days. We will probe the possible periods by scheduling CHEOPS visits on transit event windows, based on the transit probability distribution, to pin down the true period of TOI-5626.01. CHEOPS observations, combined with a currently ongoing high-precision radial velocity campaign, will allow us to solve the orbit of this long-period giant planet, and to precisely determine its mass and radius, adding a valuable target to the still sub-sampled population of warm Jupiters, and possibly identify a top-priority target for JWST atmospheric characterization of cold giants.

Measuring precise masses and radii of two key young exoplanets in the same system (PI: Hritam Chakraborty)

Young planets are excellent laboratories to test planet formation and evolution pathways. However, the accurate characterisation of parameters like the masses and radii is limited by the enhanced activity of the host star. However, the timing of individual transits is far less affected by the stellar activity. Thus, transit timing variations arising due to dynamical interactions provide a unique opportunity to measure precise masses of young exoplanets. Here we propose to characterise the TOI-942 system, a ~50-Myr-old star, hosting two Neptune-sized planets orbiting at periods of 4.32 and 10.16 days. We aim to i) constrain the precise masses of these planets by measuring their transit timing variations ii) quantify the variations in transit depth associated with stellar activity and use this information to derive accurate radii of both planets. To do this, we propose to carry out time-constrained transit observations of planets b and c, coupled with non-time-constrained monitoring of the star to better constrain the stellar activity levels. This synergy will allow us to derive both precise masses and radii of this key young exoplanetary system, making it the youngest well-characterised system of Neptune-sized planets to date.

A highly disruptive event in ASASSN-21qj: exocomets or planetary collision? (PI: Isabel Rebollido)

The environment of exoplanets is crucial for understanding their chemical and dynamical evolutionary paths. Many of the missions developed for the study of exoplanet characterisation can now be used to also target their surroundings, including small to medium bodies that could be colliding with them or evaporating and releasing material that could be accreted. The recently discovered mature dipper ASASSN-21qj is an excellent target to follow up some of these events in a solar type main-sequence star. CHEOPS unprecedented capabilities will enable the characterisation of the substructure of the dimming events, revealing size and geometry of transiting objects, and distinguish between two hypotheses: are the dimmings originated in a planetary sized collision, or are they a result of a massive cometary breakup? Combination of our results with past photometric observations (lightcurves in different wavelengths plus near to mid-IR photometry) and future JWST spectra will ensure the full characterisation of these events, and contribute greatly to understanding of planet formation and evolution processes.

Measuring Albedos for Different Hot Jupiters (PI: Prune Camille August)

Hot Jupiters offer exceptional opportunities for atmospheric study, predominantly carried out with NIR/MIR spectroscopy. However, our scientific understanding their cloud properties remains limited and has been shown to affect both transmission and emission spectra significantly. Photometric observations in the optical are necessary to provide better constraints on the reflective properties of the atmosphere, improving our understanding of clouds and hazes. We propose observing three hot Jupiters—TOI 1518 b, KELT-7 b, and HD 202772 Ab. These planets orbit bright stars (G ~ 8), making them ideal CHEOPS targets. Their similar characteristics allow exploration of how varying temperatures influence atmospheric properties, complementing existing geometric albedo data. This effort aims to complement the sample of hot Jupiters with measured geometric albedos, furthering our understanding of atmospheric dynamics across these systems and ultimately improving interpretation of their composition and evolution.

Characterizing Albedos and Eccentricities of 3 Ultra Hot Jupiters with Multi-bandpass Observations (PI: Alison Duck)

Multi-bandpass observations with TESS (Ricker et al. 2016) and CHEOPS (Benz et al. 2021) can enable us to improve our understanding of the demographics of hot Jupiter atmospheric properties. Jointly, these missions can provide detailed determinations of optimal exoplanet candidates for atmospheric follow up from the James Webb Space Telescope (JWST; Greene et al. 2016) and the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL; Tinetti et al. 2018). Thus, we propose 2 secondary eclipse observations with CHEOPS for for WASP-87 An Ultra Hot Jupiter hosting systems. This system has occultations observable by TESS, to tightly constrain its planet's physical properties, dayside temperature, albedo, heat recircularization efficiency, and orbital eccentricity to better understand the demographics of hot Jupiter atmospheric properties in highly irradiated environments.

Transit timing variations of V1298 Tau b: a step forward to decipher the elusive architecture of the infant multi-planet system V1298 Tau (PI: Pietro Leonardi)

V1298 Tau is a young (20 ± 10 Myr; mV = 10.12), pre-main sequence, early K-type star hosting at least four transiting planets (David et al. 2019b), making it an attracting multi-planet system to study the very early stages of its evolution. The aim of this proposal is to characterize in detail the orbital dynamics of V1298 Tau b, the third planet in the system in order of distance from the host star, by measuring its TTVs. The result of this characterization can provide indirect information about the whole system architecture and, as a consequence, help constraining the currently unknown orbital dynamics of V1298 Tau e.

Fresh out of the oven: A comprehensive survey of transiting young sub-Neptune planets (PI: Hinna Shivkumar)

Young planets in multi-planetary systems provide a unique insight into the freshly baked products of planet formation. By gazing into the atmospheres of these young planets, we can study their origin, evolution and ultimate fate. JWST is providing an unprecedented view into young exoplanet atmospheres. However, in order to break the degeneracies between atmospheric contribution and stellar contamination, we must widen our wavelength domain. High-precision optical photometry with CHEOPS provides the perfect setting for breaking the degeneracy between atmospheric and stellar contributions. We propose to observe five young planets that are part of JWST GO-Cycle 3 KRONOS young planet survey, namely TOI-451 c, d and TOI-2076 b, c, and d to achieve our science goals. We aim to observe three transits for each planet which will improve the optical transit depth of the planets by an order of magnitude compared to measurements from previous TESS observations